1. Field of the Invention
The present invention relates to a method for fabricating silica glass, in particular, to a method for fabricating silica glass using a sol-gel process.
2. Description of the Related Art
Some convention methods are known to fabricate glass articles of various refractive indices using a sol-gel. In particular, in optical silica glass articles such as optical fibers or lenses, the refractive index is one of the important properties of such materials. Silica glass is generally superior in regards to properties such as thermal stability and strength. Silica glass also has a relatively low coefficient of thermal expansion while being transparent and chemically inert. Due to these superior properties, silica glass is widely used in optical instruments such as optical preform or lenses.
The sol-gel process is a liquid-phase method with high productivity. This is because the sol-gel process allows composition of an article to be freely controlled and is generally preformed at a low temperature. In this regard, it offers very high economy and enables a composition with various composite components to be produced. In addition, the sol-gel process is very useful in fabricating photomasks for semiconductor, silica glass lenses or the like because the process employs high purity materials from the starting materials thereof.
The following is a description regarding a conventional procedure for fabricating silica glass using alkoxysilane.
A hydrolysis reaction is performed by adding alcohol, water, and other substances to silicon alkoxide as solvent. If the reaction is performed under an acid catalyst, a chemically cross-linked monolithic gel is obtained. If the reaction is performed under a base catalyst, a spherical colloidal silica sol is obtained. The resultant product of the hydrolysis reaction is then poured into a casting mold, thereby forming molding gel. The structure of the gel may be varied depending on a relatively contained ratio of silicon alkoxide, water, solvent, etc., or pH of hydrolyzed composition of silicon alkoxide. Thereafter, the gel is dried for a predetermined length of time and then subjected to a heat-treatment at a temperature not lower than about 700° C. As a result, a silica glass tube is obtained.
The gel formed from silicon alkoxide as described above has a problem in that it has a very high percentage of contraction after being dried because very high stress is applied to the gel due to fine pores present while the gel is dried. Accordingly, in order to prevent the gel from c racking while the gel is dried and increase yield of production, a special process is employed for controlling the level for drying the gel. In this regard, an alleviated condition (to maintain relatively low temperature and high humidity) is adopted, or the level for drying the gel is controlled using one or more small openings formed through a cover of the mold. However, because such a method requires substantial time until the drying is completed, sometimes an additive is put into the gel to enhance drying yield.
In order to fabricate silica glass, various components may be used as an additive beyond silicon alkoxide. A metal alkoxide typically added for the purpose of stable and uniform doping is made so that the silicon alkoxide and the metal are mixed and reacted during the step of hydrolysis or polymerization. The metal used in doping is not limited to a particular kind of metal. However, Al, Ti, Zr, or similar types of metals, which each have a refractive index higher than that of silica, may be used when fabricating silica glass for lenses or devices that require a relatively high refractive index.
In order to compensate for eminent difference in the hydrolysis rate between silicon alkoxide and metal alkoxide which are doped, a chelate compound is occasionally used. If a refractive index, a dispersion value, etc. of silica glass to be fabricated are determined, a proper composition for the silica glass is prepared. The silica glass is then fabricated through steps of hydrolysis reaction, polymerization reaction, fabrication of gel, drying, and heat-treatment.
If a single material is doped when silica glass is fabricated, the process of fabrication can be simplified. However, if a significantly high refractive index is required or both a high refractive index and a low dispersion are concurrently required, two or more metals may be needed. If two or more metals are doped, not only the process for fabricating uniform sol is substantially lengthened, but also very complicated calculation is required for controlling reaction rate. For example, when a composite is produced by mixing Ti(OR)4, which is a Ti precursor, and Bi(OAc)3, which is a Bi precursor, the ratio of metal dopants will be locally fluctuated and unevenness between the metal compositions will accelerate crystallization tendency depending on temperature of heat treatment.
Accordingly, what is needed in the art is an improved method.